The invention relates to a device and to a method for aligning a first element coupled to an actuator with a second element of a main system, using a position sensor and considering environmental vibrations.
In many technical systems using actuators, the performance of these actuators, and thus the overall performance of the technical systems, can be influenced by vibrations being induced to the systems and actuators from the outside as external, environmental vibrations and internal vibrations occurring at the actuator or at an element the actuator is coupled with. However, the performance of any technical systems using actuators has increased considerably and is desired to be increased further. Thus, an accurate positioning of actuators may be required to achieve an increased performance.
For example in modern tape storage systems, the capacity and performance of the tape storage systems have increased in the last years. To achieve higher cartridge or tape capacities and improved performance, however, further advances in several areas are necessary. Increases in linear and track densities on the tape may be required in order to achieve higher storage capacities. However, increase in linear densities may lead to a decrease of the distance between adjacent bit cells, which in turn may lead to an increase of inter-symbol interferences. Increase in track densities may lead to narrower individual track widths and narrower write and/or read heads which may require a very precise control of the tape transport system and track-follow control of the tape head.
To achieve the required track densities, precise positioning of the recording head over the data tracks may be required. Therefore, the performance of the track-follow control system of tape drives needs to be improved. Furthermore, operation of tape drive systems requires robust performance of the track-follow control system under shock and vibration conditions. Increasing the tape track density tightens further the tolerance in the acceptable track following error making it increasingly more challenging to meet the performance specifications under vibration conditions.
The basic function of the track-follow control system is to reduce the misalignment between the tape and the recording head created by lateral motion of the flexible medium. Lateral tape motion (LTM) arises primarily from imperfections in the tape guide rollers and reels, such as run-outs, eccentricities and other tape path imperfections.
Besides compensating for the LTM, the track-follow control system should provide an additional functionality of compensating for the external vibration disturbances. Conventionally, standard vibration profiles are used to describe the vibration specifications in terms of the acceleration input under which the tape drive must continue to operate reliably.
A track-follow control system can use a position error signal (PES) that is generated based on servo information prerecorded on the tape. The PES provides a measure of the error between the target track location on the tape and the head position. Several approaches have been proposed for improving performance under vibration condition by enhancing the PES-based track-follow controller. For example, the PES-based track-follow controller can provide an enhanced rejection at the vibration frequencies. Further, switching controllers, accelerometer measurements and disturbance observer enhancements could be used to improve the performance under vibration conditions. Controllers in this context refer to devices for controlling an actuator being responsible for actuating the head, i.e., to change the position of the head. For different vibration conditions, different controllers should be used, i.e., controllers having configurations customized to the current vibration condition. Depending on the vibration condition, the control signal for the actuator can be generated and adjusted so that the vibration condition is considered when actuating the head. However, in order to consider different vibration conditions, the kind of controller can be chosen, i.e., switching between different controllers can be performed.
Another approach for improvements in the track-following performance is to provide a higher closed-loop bandwidth, i.e., increasing the bandwidth within the control system. For higher closed-loop bandwidth, it might be necessary to either increase the physical head actuator bandwidth or increase the PES-based closed-loop bandwidth. The first approach is limited due to a large head actuator mass and power consumption or dissipation constraints. The second approach is limited due to measurement delay effects especially at low speeds and due to noise or disturbance amplification areas.
Therefore, there are several limitations in improving the track-following that is based only in the PES measurement. The same limitations applies to other technical systems using actuators wherein the PES refers to a difference between a target position of the actuator, or an element coupled to the actuator, and the actual actuator position, or element position.
Currently solutions for track-follow control systems use PES and modify the control signal which depends on the PES by considering vibration signals like LTM or by reducing noise in the system. A system using PES and considering LTM is for example disclosed in U.S. Pat. No. 8,059,362 B2. A system using PES and reducing noise in the system is for example disclosed in U.S. Pat. No. 8,068,308 B2.